Magnetoresistive sensor with antiferromagnetic exchange-coupled structure having underlayer for enhancing chemical-ordering in the antiferromagnetic layer

ABSTRACT

An antiferromagnetically exchange-coupled structure for use in a magnetic device, such as a magnetoresistive sensor, includes an underlayer formed of a chemically-ordered tetragonal-crystalline alloy, a chemically-ordered tetragonal-crystalline Mn-alloy antiferromagnetic layer in contact with the underlayer, and a ferromagnetic layer exchange-coupled with the antiferromagnetic layer. The underlayer is an alloy selected from the group consisting of alloys of AuCu, FePt, FePd, AgTi3, Pt Zn, PdZn, IrV, CoPt and PdCd, and the antiferromagnetic layer is an alloy of Mn with Pt, Ni, Ir, Pd or Rh. The underlayer enhances the transformation of the Mn alloy from the chemically-disordered phase to the chemically-ordered phase. In one example, an exchange-coupled structure with an underlayer/antiferromagnetic layer of AuCu/PtMn allows the PtMn to be made substantially thinner, thus reducing the electrical resistance of the structure and improving the performance of a current-perpendicular-to-the-plane (CPP) magnetoresistive sensor.

TECHNICAL FIELD

The invention relates generally to a ferromagnetic/antiferromagneticexchange-coupled structure, and more specifically to a magnetoresistivesensor incorporating the structure.

BACKGROUND OF THE INVENTION

The exchange-coupling of a ferromagnetic film to an adjacentantiferromagnetic film with a resulting exchange-bias field in theferromagnetic film was first reported by W. H. Meiklejohn and C. P.Bean, Phys. Rev. 102, 1413 (1959). While the magnetic hysteresis loop ofa single ferromagnetic film is centered about zero magnetic field, aferromagnetic/antiferromagnetic bilayer will show an asymmetric magnetichysteresis loop that is shifted from zero magnetic field in the plane ofthe film by an exchange-bias field, H_(B). In many cases, the directionof the exchange-bias field within the plane of the film can be setduring the growth of the antiferromagnetic film and is determined by theorientation of the magnetic moment of the ferromagnetic film when theantiferromagnetic film is deposited on top of the ferromagnetic film.The direction of the exchange bias field can also be changed by heatingthe ferromagnetic/antiferromagnetic bilayer above the so-called blockingtemperature, TB, of the antiferromagnetic film. For other cases, theantiferromagnetic film is chemically ordered, and the direction of theexchange bias field is determined by the direction of the magnetic fieldwhen an annealing step orders the antiferromagnet. The detailedmechanism that determines the magnitude of the exchange-bias field isbelieved to arise from an interfacial interaction between theferromagnetic and antiferromagnetic films.

Exchange-coupled structures have found several important applications,especially in magnetoresistive sensors used as read heads in magneticrecording hard disk drives.

The most common type of magnetoresistive sensor, called a “spin-valve”(SV) sensor, has a stack of layers that include two ferromagnetic layersseparated by a nonmagnetic electrically-conducting spacer layer. Oneferromagnetic layer, called the “pinned” layer, has its magnetizationdirection fixed by being exchange-coupled with an adjacentantiferromagnetic layer. The other ferromagnetic layer has itsmagnetization direction “free” to rotate in the presence of an externalmagnetic field, i.e., fields from the recorded data on the magneticrecording disk. With a sense current applied to the sensor, the rotationof the free-layer magnetization relative to the pinned-layermagnetization is detectable as a change in electrical resistance. Theconventional SV magnetoresistive sensor operates with the sense currentdirected parallel to the planes of the ferromagnetic layers, so it isreferred to as a current-in-the-plane (CIP) sensor. In a disk driveCIP-SV sensor or read head, the magnetization of the pinned layer isgenerally perpendicular to the plane of the disk and the magnetizationof the free layer is generally parallel to the plane of the disk in theabsence of an external magnetic field.

A SV type of magnetoresistive sensor has been proposed that operateswith sense current perpendicular to the planes (CPP) of theferromagnetic layers. CPP-SV sensors are described by A. Tanaka et al.,“Spin-valve heads in the current-perpendicular-to-plane mode forultrahigh-density recording”, IEEE TRANSACTIONS ON MAGNETICS, 38 (1):84-88 Part 1 January 2002. Another type of CPP sensor is a magnetictunnel junction (MTJ) sensor in which the nonmagnetic spacer layer is avery thin nonmagnetic insulating tunnel barrier layer. In a MTJ sensorthe tunneling current perpendicularly through the ferromagnetic layersdepends on the relative orientation of the magnetizations in the twoferromagnetic layers. While in a CPP-SV magnetoresistive read head thenonmagnetic spacer layer separating the pinned and free ferromagneticlayers is electrically conductive and is typically copper, in a MTJmagnetoresistive read head the spacer layer is electrically insulatingand is typically alumina (Al₂O₃).

The most common material used for the antiferromagnetic layer toexchange-bias the pinned ferromagnetic layer in magnetoresistive sensorsis a chemically-ordered Mn alloy with a tetragonal crystallinestructure, such as PtMn, NiMn, IrMn, PdMn and RhMn. These alloys providerelatively high exchange-bias fields, and are described in U.S. Pat. No.5,315,468. The Mn alloy material is initially chemically-disordered whendeposited and provides no exchange-biasing, but becomeschemically-ordered when annealed, as a result of thermally-activatedatomic diffusion, and then provides exchange-biasing of the pinnedferromagnetic layer.

The structure of a chemically-ordered Mn alloy with a tetragonalcrystalline structure is shown in FIG. 1 for PtMn. The Pt atoms 22 andMn atoms 24 together form a structure 20 similar to theface-centered-cubic (fcc) structure in which planes 32 of Pt atoms 22and planes 34 of Mn atoms 24 alternate along the [001] direction. Theresulting structure is termed L1₀ and corresponds to a super-lattice inthe limit that each layer is a single atomic plane thick. An axis 26perpendicular to atomic planes 32, 34 corresponds to the C-axis of L1₀structure 20, and is parallel to the [001] direction. Axes 28 areparallel to atomic planes 32, 34 and correspond to the A-axes of the L1₀structure 20. In actual devices the degree of ordered tetragonalcrystallinity varies with annealing. Complete ordering is not necessary,but sufficient ordering to obtain exchange anisotropy and stability isneeded for a robust device.

The use of these Mn alloys, particularly PtMn, as the antiferromagneticlayer in an exchange-coupled structure in a magnetoresistive sensorpresents challenges in sensor design and fabrication. These alloys mustbe made relatively thick and must be annealed at relatively hightemperatures. In CPP sensors, the large thickness of the PtMnantiferromagnetic layer is a disadvantage because the high resistivityof PtMn reduces the sensor magnetoresistance (the deltaR/R measurable bythe sensor) for a given sense current, or requires that a relativelyhigh sense current be used in the sensor to achieve the desiredmagnetoresistance. In both CIP-SV and CPP sensors, high annealtemperatures may not be compatible with the sensor fabrication process.

What is needed is an exchange-coupled structure that enables the use ofMn-alloys without adversely affecting the design and fabrication ofmagnetoresistive sensors.

SUMMARY OF THE INVENTION

The invention is an antiferromagnetically exchange-coupled structure foruse in a magnetic device, such as a magnetoresistive sensor, thatincludes an underlayer formed of a chemically-orderedtetragonal-crystalline alloy, a chemically-orderedtetragonal-crystalline Mn-alloy antiferromagnetic layer in contact withthe underlayer, and a ferromagnetic layer exchange-coupled with theantiferromagnetic layer. The underlayer is an alloy selected from thegroup consisting of alloys of AuCu, FePt, FePd, AgTi3, Pt Zn, PdZn, IrV,CoPt and PdCd, and the antiferromagnetic layer is an alloy of Mn withPt, Ni, Ir, Pd or Rh. The underlayer enhances the transformation of theMn alloy from the chemically-disordered phase to the chemically-orderedphase.

In one example, an exchange-coupled structure with anunderlayer/antiferromagnetic layer of AuCu/PtMn allows the PtMn to bemade substantially thinner, thus reducing the electrical resistance ofthe structure and improving the performance of a CPP magnetoresistivesensor.

A layer of Ru or Rh may be used as a seed layer beneath the underlayer.

Additional elements can be added to the underlayer in relatively smallamounts to provide additional properties without altering itschemical-ordering and thus its ability to enhance the transformation ofthe Mn alloy. For example, Pd can be added to relatively soft AuCu toharden it, and Fe, Pt, or Rh can be added to increase the resistivity ofAuCu to make the structure more suitable for a CIP-SV sensor.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of chemically-ordered PtMn showing thearrangement of atoms in the tetragonal L1₀ structure.

FIG. 2 is a cross-sectional view of a CPP sensor of this invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a sectional view of the CPP sensor 200 according to theinvention. Sensor 200 comprises a stack 201 of layers formed on asubstrate 202. The sensor stack 201 is located between a bottom magneticshield, shown as substrate 202, and a top magnetic shield 216. A bottomlead layer 203 is located adjacent substrate 202 and a top lead layer213 is located adjacent top shield 216. Sense current Is passes throughtop shield 216, perpendicularly through the stack 201, and throughbottom shield/substrate 202, as shown by arrows 240.

The layers in stack 201 also include a pinned ferromagnetic layer 206having a fixed magnetic moment or magnetization direction 207 orientedtransversely (out of the page), a free ferromagnetic layer 210 having amagnetic moment or magnetization direction 211 that can rotate in theplane of layer 210 in response to transverse external magnetic fields,and a nonmagnetic spacer layer 208 between the pinned layer 206 and freelayer 210. The pinned layer 206 is exchange-coupled with anantiferromagnetic layer 205 that is formed on underlayer 204. Theunderlayer 204, antiferromagnetic layer 205 and pinned ferromagneticlayer 206 together form the exchange-coupled structure 209. As a resultof the exchange-coupling with the antiferromagnetic layer 205, thepinned ferromagnetic layer 206 exhibits an exchange-bias field so thatits magnetization direction 207 will not rotate in the presence of anexternal magnetic field in the range of interest, i.e., magnetic fieldsfrom recorded data.

The pinned layer 206 and free layer 210 are typically formed of an alloyof one or more of Co, Fe and Ni, or a bilayer of two alloys, such as aCoFe—NiFe bilayer. The antiferromagnetic layer 205 is typically one ofthe chemically-ordered Mn alloys PtMn, NiMn, IrMn, PdMn or RhMn. Theseantiferromagnetic Mn alloys may also include small amounts of additionalelements, such as Cr, V, Pt, Pd and Ni, that are typically added toimprove corrosion resistance or increase electrical resistance. For aCPP-SV sensor, the spacer layer 208 is electrically conductive, and istypically formed of copper. For a MTJ sensor, the spacer layer 208 is anelectrically insulating tunnel barrier layer, and is typically alumina(Al₂O₃). Lead layers 203, 213 may be formed of Ru, Ta or a bilayer ofRu/Cu or Ta/Cu, or other well-known electrically conductive leadmaterials. The magnetic shields 202, 216 are typically formed ofpermalloy (NiFe) or sendust (FeAlSi).

The sensor 200 also includes longitudinal biasing layers 212 outside thesensor stack near the side edges of free layer 210. The biasing layers212 may be formed of hard ferromagnetic material, such as CoPt orCoCrPt, and are electrically insulated from the sensor stack 201 and theshields 202, 216 by insulating layers 214 and 218, respectively. Thebiasing layers 212 provide a longitudinal biasing magnetic field, asshown by arrows 225, to stabilize the magnetization of the free layer210 longitudinally in the direction 211 along the length of the freelayer.

While the structure shown in FIG. 1 has the pinned ferromagnetic layer206 below the free layer 210, these layers could be reversed, in whichcase the exchange-coupled structure 209 would be located above thespacer layer 208 with the order of the layers in structure 209 alsobeing reversed, i.e., pinned layer 206 would be located on spacer layer208, antiferromagnetic layer 205 would be on top of pinned layer 206 andunderlayer 204 would be on top of antiferromagnetic layer 205.

Also, the pinned layer 206 can be the well-known antiparallel-pinned(AP-pinned) structure, also called a “laminated” pinned layer, asdescribed in U.S. Pat. No. 5,465,185. The AP-pinned structure comprisesa ferromagnetic pinned film that would be in contact with theantiferromagnetic layer 205, a non-magnetic spacer film and aferromagnetic reference film. This structure minimizes magnetostaticcoupling of the pinned layer 206 with the free layer 210.

As described thus far, the sensor is like a prior art CPPmagnetoresistive sensor. However, in the CPP sensor of this inventionthe exchange-coupled structure 209 enables a substantially thinnerMn-alloy antiferromagnetic layer 205 as a result of the underlayer 204.Underlayer 204 enhances the formation of the Mn-alloy antiferromagneticlayer. The underlayer 204 is a substantially-chemically-ordered alloyhaving a tetragonal crystalline structure, the alloy being selected fromthe group consisting of alloys of AuCu, FePt, FePd, AgTi3, Pt Zn, PdZn,IrV, CoPt and PdCd.

In the preferred embodiment the antiferromagnetic layer ischemically-ordered equiatomic Pt₅₀Mn₅₀ located on and in direct contactwith an underlayer of chemically-ordered equiatomic AU₅₀CU₅₀. The twolayers are deposited by magnetron or ion-beam sputtering. After all thelayers in the sensor are deposited the sensor is subjected to annealingfor 4 hours at 250° C. As a result of thermally-activated atomicdiffusion, the AuCu underlayer transforms to the L1₀ phase and helps thePtMn with which it is in contact to also transform to the L1₀ phase.When formed on a AuCu underlayer having a thickness betweenapproximately 10 and 200 Å, the PtMn layer can be as thin as 50 Å,preferably in the range of 25 to 50 Å, and still transform to the L1₀phase and thus generate the required exchange-bias in theantiferromagnetic layer. When the PtMn is formed on a conventionalunderlayer, such as Ta or NiFeCr, it is required to be approximately 150Å thick. Thus a PtMn layer thickness of less than approximately 125 Å isa significant improvement. Because AuCu has a significantly lowerelectrically resistivity than PtMn, and because the PtMn can be madethinner, the electrical resistance of the PtMn/AuCu can be reduced by30% or more over the conventional PtMn/underlayer. This reduction inresistance increases the magnetoresistance of the sensor for a givensense current, or allows the sensor to be designed with a lower sensecurrent.

The exchange-coupled structure 209 also has advantages when used in aCIP-SV magnetoresistive sensor. A CIP-SV sensor is similar to CPP sensorshown in FIG. 2 with the primary difference being that the sense currentis in the plane of the layers in the stack 201. The biasing layers 212do not need to be insulated from the stack 201, but electricallyinsulating material, typically alumina, is located between the stack 201and the shields 202, 216. Because the electrical resistance of theantiferromagnetic layer 206, typically PtMn, is not a factor in a CIP-SVsensor, the thickness of the PtMn is less of a concern than in CPPsensors. However, as magnetic recording densities increase, the overallthickness of the sensor has to be reduced. This makes reducing the PtMnthickness also advantageous in CIP-SV sensors. For example, a sensorhaving a combined PtMn layer/underlayer thickness less thanapproximately 150 Å would be a significant improvement. Because the AuCuunderlayer encourages the transformation of the PtMn to the L1₀ phaseduring annealing, a much lower anneal temperature and/or anneal time isrequired, thereby improving the fabrication process. In a CIP-SV sensor,shunting of current from the additional AuCu or other layer would leadto reduced magnetoresistance. To reduce or prevent this effect an alloywith a higher resistivity, such as FePt or AgTi3, or ternary alloys ofAg, Ti, Pd, V, could be used.

While AuCu is the preferred underlayer material, other materials thatcan be expected to provide similar enhancement of the transformation ofthe Mn-alloy to the L1₀ phase are FePt, FePd, AgTi3, Pt Zn, PdZn, IrV,CoPt and PdCd because each of these materials is also in the L1₀ orrelated structure with a very close lattice parameters to PtMn.

The grain size of AuCu is about 90 Å in the plane while the grain sizeof PtMn is about 120 Å. Thus it is believed that the enhancingproperties of the AuCu underlayer can be improved by depositing the AuCuon a seed layer of Ru or Rh. Ruthenium and rhodium have hexagonal-close(hcp) cell lattices relatively close to the hcp surface equivalentlattice for as-deposited fcc AuCu, and thus should promote theappropriate texture and larger grain size in the AuCu.

In the preferred embodiment of the chemically-ordered film, the Pt andMn in the PtMn antiferromagnetic layer, and the Au and Cu in theenhancing underlayer, are present in “generally equiatomic” amounts,i.e., when the atomic percentage of either the first or second elementis present in a two-element film in the range of approximately 35-65atomic percent. The existence range in compositions of internetalliccompounds (ordered intermetallic phases like PtMn, AuCu etc.) isgenerally fairly broad, approximately +/−15%. This has to do with theweak nature of the metallic bond, as compared with the ionic bond insemiconductors where typically only very narrow “line” compounds can beformed. The weak metallic bond allows for segregation and diffusion.

Additional elements can be added to the underlayer in relatively smallamounts, typically less than 10 atomic percent, to provide additionalproperties without altering its chemical-ordering and thus its abilityto enhance the transformation of the Mn-alloy to the L1₀ phase. Forexample, because undesirable smearing of soft layers can occur duringmechanical lapping of the head carrier or slider on which the sensor isfabricated, and AuCu is a soft alloy, Pd can be added in an amount lessthan approximately 10 atomic percent to harden the AuCu alloy. Also, Fe,Pt, or Rh will increase the resistivity of the AuCu layer, making itmore suitable for CIP-SV applications.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

1. An antiferromagnetically exchange-coupled structure in a magneticdevice of the type having a substrate and a plurality of ferromagneticlayers, the structure being formed on the substrate and comprising: anunderlayer formed of a substantially-chemically-ordered alloy having atetragonal crystalline structure, the alloy selected from the groupconsisting of alloys of AuCu, FePt, FePd, AgTi3, Pt Zn, PdZn, IrV, CoPtand PdCd; an antiferromagnetic layer in contact with the underlayer andformed of a substantially-chemically-ordered alloy comprising X and Mnand having a tetragonal crystalline structure, wherein X is selectedfrom the group consisting of Pt, Ni, Ir, Pd and Rh; and a ferromagneticlayer exchange-coupled with the antiferromagnetic layer.
 2. Thestructure of claim 1 further comprising a seed layer consistingessentially of Ru or Rh, the underlayer being located on the seed layer.3. The structure of claim 1 wherein the underlayer alloy furthercomprises one or more of Pd, Fe, Pt, and Rh.
 4. The structure of claim 1wherein the antiferromagnetic alloy further comprises one or more of Cr,Pt, Pd, V and Ni.
 5. The structure of claim 1 wherein the first elementlisted in each underlayer alloy in the group is present in the alloy inamount between approximately 35 and 65 atomic percent.
 6. The structureof claim 1 wherein the underlayer alloy comprises Au and Cu and theantiferromagnetic alloy comprises Pt and Mn.
 7. The structure of claim 6wherein the thickness of the PtMn alloy antiferromagnetic layer is lessthan approximately 125 Angstroms.
 8. The structure of claim 7 whereinthe thickness of the AuCu underlayer is between approximately 10 and 200Angstroms.
 9. A magnetoresistive read head for sensing data recorded ona magnetic recording medium in the presence of sense current through thehead, the head comprising: a substrate; an exchange-coupled structure onthe substrate and comprising (a) an underlayer formed of asubstantially-chemically-ordered alloy having a tetragonal crystallinestructure, the alloy selected from the group consisting of alloys ofAuCu, FePt, FePd, AgTi3, Pt Zn, PdZn, IrV, CoPt and PdCd; (b) anantiferromagnetic layer in contact with the underlayer and formed of asubstantially-chemically-ordered alloy comprising X and Mn and having atetragonal crystalline structure, wherein X is selected from the groupconsisting of Pt, Ni, Ir, Pd and Rh; and (c) a pinned ferromagneticlayer exchange-coupled with the antiferromagnetic layer and having amagnetization direction oriented substantially perpendicular to theplane of the recording medium and substantially prevented from rotatingin the presence of magnetic fields from the recording medium; a freeferromagnetic layer having a magnetization direction orientedsubstantially parallel to the plane of the recording medium in theabsence of an external magnetic field, said free layer magnetizationdirection being substantially free to rotate in the presence of magneticfields from the recording medium; and a nonmagnetic spacer layer betweenthe pinned ferromagnetic layer and the free ferromagnetic layer.
 10. Thehead according to claim 9 wherein the free layer is located between thesubstrate and the exchange-coupled structure.
 11. The head according toclaim 9 wherein the head is a current-in-the-plane head having the sensecurrent directed substantially parallel to the plane of the free layer.12. The head according to claim 9 wherein the head is acurrent-perpendicular-to-the-plane head having the sense currentdirected substantially perpendicular to the plane of the free layer. 13.The head according to claim 12 wherein the head is a spin-valve head andwherein the nonmagnetic spacer layer is electrically conducting.
 14. Thehead according to claim 12 wherein the head is a magnetic tunneljunction head and wherein the nonmagnetic spacer layer is anelectrically-insulating tunnel barrier.
 15. The head of claim 12 furthercomprising a seed layer consisting essentially of Ru or Rh, theunderlayer being located on the seed layer.
 16. The head of claim 12wherein the underlayer alloy further comprises one or more of Pd, Fe,Pt, and Rh.
 17. The head of claim 12 wherein the antiferromagnetic alloyfurther comprises one or more of Cr, Pt, Pd, V, and Ni.
 18. The head ofclaim 12 wherein the first element listed in each underlayer alloy inthe group is present in the alloy in amount between approximately 35 and65 atomic percent.
 19. The head of claim 12 wherein the underlayer alloycomprises Au and Cu and the antiferromagnetic alloy comprises Pt and Mn.20. The head of claim 19 wherein the thickness of the PtMn alloyantiferromagnetic layer is less than approximately 125 Angstroms. 21.The head of claim 20 wherein the thickness of the AuCu underlayer isbetween approximately 10 and 200 Angstroms.